The present invention relates to a digital synthesizer of signals. It can be applied especially to direct digital frequency synthesis, for example in the field of radar technology or in that of instrumentation.
Direct digital synthesis is a technique of frequency synthesis by which the samples of a signal to be generated are prepared in tile form of digital values and then converted into analog signal form by means of a digital-analog converter. The signal synthesizers obtained by this technique are highly valuable in terms of volume, weight and energy consumption because they enjoy the advantage of large-scale integration. Their other particular advantages are very high resolution and very short times of switching from one frequency to another.
In general, the digital-analog converter has a resolution of less than the number N of useful digital bits of the signal that is prepared for conversion into an analog signal. The passage from N bits to M bits, which is the resolution of the digital-analog converter, generates a quantification error. Owing to this quantification, the transfer function of the digital-analog converter, namely the output voltage from the converter as a function of the input digital words, is a stepped function.
In these digital synthesizers, the spectral purity is furthermore limited by the non-linearities of the digital-analog converters that they contain. The non-linearities designate the fact that the steps of the transfer function of the digital-analog converter are not of equal height and that the transition between steps produces irregular phenomena. These non-linearities lead to the generation of harmonic frequencies that are aliased owing to the sampling. Parasitic lines are thus created, a parasitic line being a spectral component located at frequencies different from the fundamental frequency of the signal to be synthesized.